Experimental study of disordered mica structures by high-resolution electron microscopy

Disorder in stacking sequences of mica minerals, predominantly 1M muscovite from York, Ontario and biotite from Mitchell Co., North Carolina, was observed using high-resolution electron microscopy. The specimens were prepared by sectioning the flakes of mica in the microtome with a diamond knife, so that crystals were viewed down the direction parallel to the Si-O layers. Disordered sequences of the layers, as well as ordered crystals, are best described by citing the positions of tunnels between alkali ions lying in the interlayers, since these individual sites are resolved in electron micrographs of micas. An evaluation of the usefulness of one-dimensional lattice fringe images for studying disordered states in crystals is also discussed by comparing them with structure images of corresponding crystals. Intimate intergrowths of different mica polytypes on a scale of tens of ångströms raise questions as to the definition of origins of unit cells and therefore polytypes.